1. Technical Field
A method of manufacturing semiconductor devices is disclosed which can inhibit TED (transient enhanced diffusion) of dopant implanted into the bottom structure and which can also limit degradation of the oxide film quality at the upper side due to outgasing that can occur during the annealing after the ion implantation is implemented.
2. Discussion of the Related Art
In the manufacture of semiconductor devices, ion implantation processes, deposition processes and etching processes are all used.
In the manufacture flash memory devices or transistors, a well region is formed by means of an ion implantation process. An ion implantation layer for controlling the threshold voltage is then formed at a given depth of the well. Next, before a pad nitride film is formed, a tunnel oxide film and a first polysilicon layer for forming a floating gate is formed and is then patterned.
Thereafter, the structure is etched to form a trench. A wall sacrificial oxidation process and a well oxidation process are then sequentially implemented to form an isolation film by means of a SA-STI (self-aligned shallow trench isolation) method for electrically isolating the devices.
In the above process, in case of a flash memory device using an nMOS transistor as a cell, boron (B) is implanted to form an ion implantation layer for adjusting the threshold voltage. At this time, as the cell is programmed and erased in a sector program/erase mode of 512 byte unit of the flash device, the threshold voltages of the cells need to be uniform within the unit cell block.
When using FN (Fowler-Nordheim) tunneling and not HCE (hot carrier effect) as a program mode, distribution of the dopant to control the threshold voltage becomes an important parameter. For this reason, it is important that the distribution of the ion implantation layer not be changed by the subsequent annealing process so that the operation speed can be increased by controlling the threshold voltage.
If the isolation film is formed by the SA-STI method, however, a transient enhanced diffusion (TED) of the dopant occurs during the high temperature oxidization process, which changes the threshold voltage of the device. Furthermore, if a large quantities of dopant are implanted to adjust the threshold voltage of the device, RDG (remained dopant gettering) may occur due to the large quantities of dopant used. RDG can cause device failure. Therefore, in order to minimize damage caused by ion implantation, high temperature annealing must be implemented as a subsequent process which limits the amount of dopant that can be used to control the threshold voltage.
In addition, in the case of BF2 which is widely used as a dopant when forming shallow surface channels, dopant loss occurs due to outgasing caused by the subsequent annealing process. In particular, fluorine-induced (F-induced) outgasing during annealing is inevitable due to the large quantities of fluorine (F) present and its tendancy to be outgassed. As a result, the oxide film quality is degraded.